Artifact-free calibration of spatial carrier interferometry

Abstract

Spatial carrier interferometry is a well-known single frame wavefront phase measuring technique. In this technique a large relative tilt is placed between the test and reference beams producing a high frequency carrier fringe pattern which is modulated by the desired measurement wavefront. Implementation of spatial carrier interferometry is relatively easily accomplished on most advanced laser interferometers. Since it is a single frame technique, it provides robust vibration immunity, enabling measurements involving long paths or mechanically decoupled elements as well as metrology into vacuum chambers and overall environmental immunity. One of the major limitations of this technique is the degradation in measurement accuracy resulting from the large wavefront tilt applied between the test and reference beams. As a result of the large relative beam angle, the test and reference beams do not follow exactly the same path through the interferometer, resulting in what is generally known as retrace error. In this paper an automated calibration technique is introduced which determines the retrace error in a measurement setup without the use of a calibration artifact. This technique works well when measuring both flat and spherical test surfaces. In both cases, the difference between the calibrated wavefront and the wavefront measured on-axis with temporal phase shifting is less than .05 waves. This process allows nanometer-level measurement of precision optics even in difficult environments.